Okay, let's think of it like this: you know how sometimes you want to do something or say something, but your mom or dad or teacher tells you that you can't because it's not appropriate or it might hurt someone's feelings? That's kind of what book censorship is.
When grown-ups or people in charge decide that they don't want other people to read certain books because they think that the books might be bad or harmful in some way, they censor the books. It's like they put a big hand over the book and say "No, you can't read this."
People who are against book censorship say that everyone should be able to read whatever books they want, as long as they can understand them. They say that it's not fair to keep information from people or to tell them what they can or can't think about certain things.
But people who are for book censorship say that some books have bad things in them like violence or bad ideas, and that those things could hurt people or make them do bad things too. They say that it's better to not let people read those books than to let them read something that could be harmful.
So basically, book censorship is when some grown-ups or people in charge decide what books you are allowed to read and what books you aren’t allowed to read. Some people think it's good, while others think it's bad.